The present invention relates to metal powder compositions and more particularly to a method for preventing the segregation of powders of differing specific gravities.
Admixed powders having differing specific gravities tend to segregate during movement, flow or vibration. Such segregation is undesirable because subsequent compaction and sintering of a segregated powder mix may well produce final products having imperfections and non uniform physical characteristics. This problem arises frequently in the formation of powder metal parts where the basic powder metal is low carbon steel, stainless steel, tool steel, high alloy steel or, in fact, any steel where a low specific gravity powder such as graphite is added. As more fully explained in U.S. Pat. No. 4,121,927, the graphite is used to remove or limit the formation of oxides from the powder metal and also to provide the alloying element for the final composition of the metal product.
The graphite, having a much lower specific gravity than the metallic powder with which it is mixed, tends to segregate during any movement or flow of the mixture or even from vibrations during storage. Segregation even occurs during compaction. The present invention provides a method for eliminating such segregation. The present method is also effective for preventing segregation from metal powder of such additives such as silicon, boron, phosphorus or other powders which have a lower specific gravity than the metallic powder with which it is being mixed.
It is known to add a lubricant such as Acrawax (trademark), in powder form, to the admixed powders to provide lubrication during the compacting process. The usual amount of such lubricant is about 1% by weight of the total powder.
When small amounts (up to 1% by weight) of lower specific gravity powder such as graphite are admixed with the metallic powder, the wax lubricant helps to prevent segregation. But when higher percentages of powder additives are necessary, segregation will occur despite the wax lubricant.
Accordingly, it is an object of the present invention to prevent the segregation of lower specific gravity powders added to a metallic powder.
The present invention prevents the segregation of lower specific gravity powders such as graphite when added to a metallic powder. In the method of the present invention, furfuryl alcohol is mixed with the powder metal and the lower specific gravity powder. During mixing, an acid such as toluene sulfonic acid is added to convert the alcohol to a solid resin film on the powder metal particles. The cured resin acts as a binder to bond the lighter powders to the metal powder and virtually eliminate all possibility of segregation.
The concentration and the amount of the acid can be adjusted so that the polymerization of the alcohol takes place while the blend is in motion during mixing. By the present method, the powder metal blend remains dry and free flowing, and the furfuryl alcohol resin does not bind the powder into a solid mass. The flowability of the powder blend of the present invention has proven superior to that of a powder metal blend utilizing a wax lubricant. However, wax lubricants can be used with the blend of the present invention for die lubrication purposes.
Since furfuryl alcohol contains a large amount of carbon, approximately one-half of the alcohol added is converted to carbon, and is dissolved in the metal powder along with the lower specific gravity powder, which is graphite in many cases, during subsequent sintering after compaction.